SEASONAL PATTERNS AND ENVIRONMENTAL-REGULATION OF FROST HARDINESS IN SHOOTS OF SEEDLINGS OF THUJA-PLICATA, CHAMAECYPARIS-NOOTKATENSIS, AND PICEA-GLAUCA

Seasonal patterns of frost hardiness and the effects of photoperiod, w ater stress, and low temperature on hardiness development were examine d in shoots of Ist-year seedlings of western red cedar (Thuja plicata Donn), yellow cedar (Chamaecyparis nootkatensis (D. Don) Spach), and w hite spruce (Picea glauca (Moench) Voss). Under natural conditions, sp ruce hardened in two stages starting in early August, first at a slow rate in response to shortening photoperiod, then at a faster rate in r esponse to decreasing temperatures. Western red cedar and yellow cedar started hardening in late October to early November when the photoper iod was already less than 11 h and daily maximum temperatures less tha n 15 degrees C. Seedlings of these two species attained a greater degr ee of frost hardiness when,exposed to persistent subfreezing temperatu res. Under controlled conditions, 4 weeks of short photoperiod at warm temperatures (9 h light : 15 h dark, 20:15 degrees C) had little effe ct on hardiness of the two cedars but increased hardiness in spruce to about -15 degrees C. Water stress increased hardiness in spruce under long photoperiod (18 h light : 6 h dark, 20:15 degrees C) but only ma rginally in western red and yellow cedar. Low temperatures (7:3 degree s C, light:dark, 9-h photoperiod) increased hardiness in all species. Exposure to 2 and -3 degrees C (light:dark, 9-h photoperiod) increased the rate of hardening in western red and yellow cedar but not in whit e spruce. The ability to deharden in white spruce was related to chill ing requirements and not necessarily exposure to warm temperatures. Th e induction and maintenance of hardiness in the three species appear t o be controlled by different environmental signals.